Her full backstory is not revealed, but she states that her spirit was trapped in a strange Death Mask found in a local haunted house for around a hundred years. She was so lonely that her cries were heard by a boy named Scott Harmon who was compelled to open up the box containing the mask. By taking the mask, he freed her spirit. He found that when he put it on, he was transported to a fantasy world which looked like ours but weird things happened such as his family having cow tongue for dinner.
One weekend, his Aunt Ida was supposed to visit so Lillian came to his house pretending to be her. She snooped around looking for the mask to get it back. She was not able to find it and had to leave when Scott's mom returned. On Halloween night, Scott, his friend Amanda came back to the house hoping to scare two bullies named Mickey and Morty Klass. They wanted Scott's sister Rita to disguise herself as Lillian but soon the real Lillian appeared and claimed that Rita is fine where she is.
She explained that her spirit is tied to the mask so that she can't be fully freed from it unless someone else is trapped in it, and she chooses Scott. She put the mask on him but he fought back and was able to put it back on her, trapping her in it once again.
Scott wasn't able to find Rita but when he got home, she was there. However, she acted weirdly and had a gold tooth, just like Lillian...
She has a powdery and pale face with wavy white hair, skin drawn tight against her cheeks, a gold tooth, and gray lips. The only color on her face is that of her bright blue eyes. In HorrorTown, Lillian has purple skin with red snakes for hair. She also wears a black dress with a green brooch around her neck.